Chalcogenide optical fiber links for quantum communications systems and methods of storing and releasing photons using the same

ABSTRACT

A quantum memory system includes a chalcogenide optical fiber link, a magnetic field generation unit and a pump laser. The chalcogenide optical fiber link includes a photon receiving end opposite a photon output end and is positioned within a magnetic field of the magnetic field generation unit when the magnetic field generation unit generates the magnetic field. The pump laser is optically coupled to the photon receiving end of the chalcogenide optical fiber link. The chalcogenide optical fiber link includes a core doped with a rare-earth element dopant. The rare-earth element dopant is configured to absorb a storage photon traversing the chalcogenide optical fiber link upon receipt of a first pump pulse output by the pump laser. Further, the rare-earth element dopant is configured to release the storage photon upon receipt of a second pump pulse output by the pump laser.

This application is a continuation of U.S. application Ser. No.15/203,292, filed on Jul. 6, 2016, which claims the benefit of U.S.Provisional Application Ser. No. 62/298,706, filed on Feb. 23, 2016, thecontent of which are relied upon and incorporated herein by reference inits entirety, and benefit of priority under 35 U.S.C. §120 is herebyclaimed.

BACKGROUND

The present disclosure relates to vacuum assisted quantum memory systemsand quantum repeater systems. More specifically, the present disclosureintroduces technology for quantum memory systems and quantum repeatersystems having a chalcogenide optical fiber link.

BRIEF SUMMARY

According to the subject matter of the present disclosure, a quantummemory system includes a chalcogenide optical fiber link, a magneticfield generation unit and a pump laser. The chalcogenide optical fiberlink includes a photon receiving end opposite a photon output end. Thechalcogenide optical fiber link is positioned within a magnetic field ofthe magnetic field generation unit when the magnetic field generationunit generates the magnetic field. The pump laser is optically coupledto the photon receiving end of the chalcogenide optical fiber link. Thechalcogenide optical fiber link includes a core doped with a rare-earthelement dopant. The rare-earth element dopant is configured to absorb astorage photon traversing the chalcogenide optical fiber link when (i)the storage photon transfers an electron of the rare-earth elementdopant from a first split ground state to an excited energy state and(ii), upon receipt of a first pump pulse output by the pump laser, thefirst pump pulse transfers the electron of the of the rare-earth elementdopant from the excited energy state into a second split ground state.Further, the rare-earth element dopant is configured to release thestorage photon when (i) the electron of the of the rare-earth elementdopant is transferred from the second split ground state to the excitedenergy state, upon receipt of a second pump pulse output by the pumplaser and (ii) the electron of the rare-earth element dopant decays fromthe excited energy state to the first split ground state such that thestorage photon exits the photon output end of the chalcogenide opticalfiber link.

In accordance with one embodiment of the present disclosure, a quantumrepeater system includes two chalcogenide optical fiber links, one ormore magnetic field generation units, one or more pump lasers, andentanglement optics. Each chalcogenide optical fiber link includes aphoton receiving end opposite a photon output end. Each chalcogenideoptical fiber link is positioned within a magnetic field of the one ormore magnetic field generation units when the one or more magnetic fieldgeneration units generate the magnetic field. At least one of the one ormore pump lasers are optically coupled to the photon receiving end ofeach chalcogenide optical fiber link. Each chalcogenide optical fiberlink includes a core doped with a rare-earth element dopant. Therare-earth element dopant is configured to absorb a storage photontraversing the chalcogenide optical fiber link when (i) the storagephoton transfers an electron of the rare-earth element dopant from afirst split ground state to an excited energy state and (ii), uponreceipt of a first pump pulse output by the one or more pump lasers, thefirst pump pulse transfers the electron of the of the rare-earth elementdopant from the excited energy state into a second split ground state.Further, the rare-earth element dopant is configured to release thestorage photon when (i) the electron of the of the rare-earth elementdopant is transferred from the second split ground state to the excitedenergy state, upon receipt of a second pump pulse output by the one ormore pump lasers and (ii) the electron of the rare-earth element dopantdecays from the excited energy state to the first split ground statesuch that the storage photon exits the photon output end of thechalcogenide optical fiber link. Moreover, the entanglement opticsinclude two entangling pathways optically coupled to the photon outputend of each chalcogenide optical fiber link a beamsplitter positionedsuch that each entangling pathway traverses the beamsplitter.

In accordance with another embodiment of the present disclosure, anentangled photon generator includes a first quantum repeater system, asecond quantum repeater system, entanglement optics, a pathway splitter,and an entanglement detector. The first quantum repeater system and thesecond quantum repeater system each include two chalcogenide opticalfiber links. The first quantum repeater system and the second quantumrepeater system are each structurally configured to generate twoentangled pairs of photons. Further, the entanglement optics include afirst entangling pathway optically coupled to and extending between thefirst quantum repeater system and the entanglement detector and a secondentangling pathway optically coupled to and extending between the secondquantum repeater system and the pathway splitter.

In accordance with another embodiment of the present disclosure, amethod of absorbing and releasing a storage photon in a quantum memorysystem includes generating a magnetic field using a magnetic fieldgeneration unit and emitting a storage photon from a storage photongenerator optically coupled to a photon receiving end of a chalcogenideoptical fiber link. The chalcogenide optical fiber link includes aphoton output end opposite the photon receiving end. The chalcogenideoptical fiber link is positioned within the magnetic field generated bythe magnetic field generation unit. The chalcogenide optical fiber linkincludes a core doped with a rare-earth element dopant and upon receiptof the storage photon by the chalcogenide optical fiber link, thestorage photon is absorbed within the core doped with the rare-earthelement dopant by transferring an electron of the rare-earth elementdopant from a first split ground state to an excited energy state. Themethod also includes emitting a first pump pulse from a pump laseroptically coupled to the photon receiving end of the chalcogenideoptical fiber link such that the first pump pulse transfers the electronof the rare-earth element dopant from the excited energy state to asecond split ground state, upon receipt of the first pump pulse by thechalcogenide optical fiber link, to store the storage photon within thecore doped with the rare-earth element dopant. The method furtherincludes emitting a second pump pulse from the pump laser such that (i)the second pump pulse transfers the electron of the rare-earth elementdopant from the second split ground state to the excited energy state,upon receipt of the first pump pulse by the chalcogenide optical fiberlink, and (ii) the electron of the rare-earth element dopant decays fromthe excited energy state to the first split ground state such that thestorage photon exits the photon output end of the chalcogenide opticalfiber link.

In accordance with another embodiment of the present disclosure, aquantum memory system includes a chalcogenide optical fiber link, amagnetic field generation unit and a pump laser. The chalcogenideoptical fiber link includes a photon receiving end opposite a photonoutput end. The chalcogenide optical fiber link is positioned within amagnetic field of the magnetic field generation unit when the magneticfield generation unit generates the magnetic field. The pump laser isoptically coupled to the photon receiving end of the chalcogenideoptical fiber link. Further, the chalcogenide optical fiber linkincludes a core doped with a rare-earth element dopant configured tostore a storage photon for a photon storage lifetime comprising betweenabout 1 ns and about 1 μs.

In accordance with yet another embodiment of the present disclosure, aquantum memory system comprising a chalcogenide optical fiber link, amagnetic field generation unit and a pump laser. The chalcogenideoptical fiber link includes a photon receiving end opposite a photonoutput end. The chalcogenide optical fiber link is positioned within amagnetic field of the magnetic field generation unit when the magneticfield generation unit generates the magnetic field. The pump laser isoptically coupled to the photon receiving end of the chalcogenideoptical fiber link. Further, the chalcogenide optical fiber linkcomprises a core doped with a rare-earth element dopant configured toabsorb about 50% or more of a plurality of storage photons traversingthe chalcogenide optical fiber link.

Although the concepts of the present disclosure are described hereinwith primary reference to some specific vacuum assisted wound closureassembly configurations, it is contemplated that the concepts will enjoyapplicability to quantum memory systems and quantum repeater systemshaving any configuration.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a schematic illustration of a quantum memory system having achalcogenide optical fiber link with a rare-earth element dopant,according to one or more embodiments shown and described herein;

FIG. 2 is a schematic illustration of ground and excited energy statesof an electron of the rare earth element dopant of FIG. 1, according toone or more embodiments shown and described herein;

FIG. 3 is a schematic illustration of a quantum repeater system havingmultiple chalcogenide optical fiber link with a rare-earth elementdopant, as depicted in FIG. 1, according to one or more embodimentsshown and described herein; and

FIG. 4 schematically depicts an example entangled photon generatorcomprising the quantum repeater system of FIG. 3, according to one ormore embodiments shown and described herein.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of quantum memory system 100. Thequantum memory system 100 comprises a chalcogenide optical fiber link120, a magnetic field generation unit 150, a storage photon generator170, and a pump laser 180. As described below, the quantum memory system100 is structurally configured to store and release one or more storagephotons, for example, on demand, such that the quantum memory system 100may be synchronized with one or more additional quantum memory systemsto form a quantum repeater system 200, for example, as depicted in FIG.3. Further, the quantum repeater system 200 of FIG. 3 may be may bestructurally configured to entangle a pair of storage photons that areeach stored and released by respective quantum memory systems. Moreover,the quantum memory system 100 and the quantum repeater system 200described herein may be incorporated into one or more quantumcommunications systems, for example, quantum key generation systems,quantum telecommunications systems, quantum internet systems, and anyother current or yet-to be developed quantum communications systems.

As depicted in FIG. 1, the chalcogenide optical fiber link 120 of thequantum memory system 100 includes a core 122 doped with a rare-earthelement dopant 130 and a photon receiving end 126 opposite a photonoutput end 128. The chalcogenide optical fiber link 120 also includes acladding layer 124 surrounding the core 122. The chalcogenide opticalfiber link 120 may comprise a diameter of between about 75 μm and about200 μm, such as about 100 μm, 125 μm, 150 μm, 175 μm, or the like, asdefined by the required optical depth of the chalcogenide optical fiberlink 120. The chalcogenide optical fiber link 120 may also comprise alength of between about 0.25 m and about 10 m, for example, about 0.5 m,1 m, 2.5 m, 5 m, 7.5 m, or the like, as defined by the required opticaldepth of the chalcogenide optical fiber link 120. Further, thechalcogenide optical fiber link 120 comprises glass that includes achalcogen material, such as sulfur, selenium, tellurium, or combinationsthereof.

The rare-earth element dopant 130 doped into the core 122 of thechalcogenide optical fiber link 120 includes one or more rare-earthelements, for example, one or more lanthanide elements, includingerbium, thulium, and praseodymium, as well non-lanthanide elements suchas scandium and yttrium. Further, the rare-earth element dopant 130 maycomprises between about 0.01% and about 2% of the total molecular weightof the chalcogenide optical fiber link 120, for example, 0.025%, 0.05%,0.075%, 0.1%, 0.125%, 0.25%, 0.5%, 0.75%, 1.0%, 1.25%, 1.5%, 1.75%, orthe like. The rare-earth element dopant 130 doped into the core 122 thechalcogenide optical fiber link 120 also includes a plurality ofelectrons, each comprising a plurality of energy states. Further, theplurality of electrons are transferable between energy states, forexample, when the core 122 of the chalcogenide optical fiber link 120receives one or more storage photons emitted by the storage photongenerator 170 and/or one or more pump pulses emitted by the pump laser180, as described in more detail below.

The storage photon generator 170 is optically coupled to the photonreceiving end 126 of the chalcogenide optical fiber link 120 and isstructurally configured to generate and emit a storage photon, forexample, an entangled storage photon or a non-entangled storage photon.The storage photon generator 170 comprises a photon source, for examplea laser, a laser optically coupled to a non-linear crystal, a parametricdown convertor, or the like. Further, the storage photon generator 170may generate and emit storage photons using a four-wave mixing process,or any method or process of generating photons.

In operation, the storage photon generator 170 may generate and emitstorage photons having any wavelength, for example, between about 500 nmand about 2200 nm, for example about 1550 nm. As a non-limiting example,the storage photon emitted by the storage photon generator 170 maycomprise a first entangled storage photon that is entangled with asecond entangled storage photon simultaneously emitted by the storagephoton generator 170. In operation, the first entangled storage photonmay traverse the chalcogenide optical fiber link 120 and the secondentangled storage photon may travel along a pathway separate from thechalcogenide optical fiber link 120 while remaining entangled with thefirst entangled storage photon.

Referring still to FIG. 1, the storage photon generator 170 may beoptically coupled to the chalcogenide optical fiber link 120 using astorage photon transmission fiber 172 or other waveguide device, whichmay extend between the storage photon generator 170 and the photonreceiving end 126 of the chalcogenide optical fiber link 120. Further,the storage photon generator 170 may be optically coupled to the photonreceiving end 126 of the chalcogenide optical fiber link 120 by aligningthe storage photon generator 170 with the photon receiving end 126, forexample, using one or more alignment mechanisms 142 structurallyconfigured to optically align the storage photon generator 170 with thecore 122 of the chalcogenide optical fiber link 120. The one or morealignment mechanisms 142 may comprise an alignment stage, an opticalswitch, or both. Further, the storage photon generator 170 and/or thechalcogenide optical fiber link 120 may be coupled to individualalignment mechanisms 142.

The pump laser 180 is optically coupled to the photon receiving end 126of the chalcogenide optical fiber link 120 and is structurallyconfigured to generate and emit pump pulses. The pump laser 180 maycomprise any laser source, for example, a diode laser, an externalcavity diode laser, a fiber laser, a dye laser, or the like. Further,the pump laser 180 may be structurally configured to emit pump pulseshaving any wavelength, for example, between about 500 nm and about 2200nm. Moreover, the wavelength of the pump pulses generated and emitted bythe pump laser 180 may be larger than the wavelength of the storagephotons generated and emitted by the storage photon generator 170.

As depicted in FIG. 1, the pump laser 180 may be optically coupled tothe chalcogenide optical fiber link 120 using a pump pulse transmissionfiber 182 or other waveguide device, which may extend between the pumplaser 180 and the photon receiving end 126 of the chalcogenide opticalfiber link 120. Further, the pump laser 180 may be optically coupled tothe photon receiving end 126 of the chalcogenide optical fiber link 120using one or more alignment mechanisms 142 structurally configured tooptically align the pump laser 180 with the core 122 of the chalcogenideoptical fiber link 120. The one or more alignment mechanisms 142 maycomprise an alignment stage, an optical switch, or both. Further, thepump laser 180 and/or the chalcogenide optical fiber link 120 may becoupled to individual alignment mechanisms 142.

Referring still to FIG. 1, the magnetic field generation unit 150 maycomprise any magnetic device structurally and compositionally configuredto generate a magnetic field, for example, a static magnetic field. Asnon-limiting examples, the magnetic field generation unit 150 maycomprise an electromagnet, a ferromagnet, an alcnico magnet, a samariumcobalt (SmCo) magnet, a neodymium iron boron (NdFeB) magnet, orcombinations thereof. Further, the magnetic field generation unit 150 ispositioned within the quantum memory system 100 such that, when themagnetic field generation unit 150 generates a magnetic field, thechalcogenide optical fiber link 120 is positioned within the magneticfield of the magnetic field generation unit 150. For example, themagnetic field generation unit 150 may be adjacent the chalcogenideoptical fiber link 120. As a non-limiting example, the magnetic fieldgeneration unit 150 may be structurally and compositionally configuredto generate a magnetic field comprising a magnetic flux density ofbetween about 0.2 tesla and about 5 tesla, such as about 0.4 tesla, 0.5tesla, 0.6 tesla, 0.65 tesla, 0.7 tesla, 0.8 tesla, 1 tesla, 2 tesla,2.5 tesla, 3 tesla, 4 tesla, or the like.

As schematically depicted in FIG. 2, when the chalcogenide optical fiberlink 120 is positioned within the magnetic field of the magnetic fieldgeneration unit 150, a ground state of the plurality of electrons of therare-earth element dopant 130 is split such that each electron of therare-earth element dopant 130 comprises a first split ground state G₁, asecond split ground state G₂ and an excited energy state E₁. Bysplitting the ground state of the electrons of the rare-earth elementdopant 130, the electron may be transferred into the second ground stateG₂ to store the storage photon within the chalcogenide optical fiberlink 120, as described below.

Referring again to FIG. 1, the quantum memory system 100 may furthercomprise a wavelength division multiplexer (WDM) 160 positioned adjacentand optically coupled to the photon output end 128 of the chalcogenideoptical fiber link 120. The WDM 160 may be optically coupled to both astorage photon pathway 162 and a pump pulse pathway 164, for example,the WDM 160 may be positioned between the photon output end 128 and boththe storage photon pathway 162 and the pump pulse pathway 164. The WDM160 is configured to direct the storage photons into the storage photonpathway 162 and direct the pump pulses into the pump pulse pathway 164.For example, the WDM 160 may direct a wavelength range of photonsencompassing the wavelengths of the storage photons into the storagephoton pathway 162 and may direct a wavelength range of photonsencompassing the wavelengths of the pump pulses into the pump pulsepathway 164. Further, the storage photon pathway 162 and the pump pulsepathway 164 may comprise optical fibers.

The storage photon pathway 162 may extend between the WDM 160 and astorage photon receiver 166. As one non-limiting example, the storagephoton receiver 166 may comprise an optical fiber link of one or morephoton entanglement chains of the quantum key generation systemdescribed in U.S. patent application Ser. No. 14/680,522. As anothernon-limiting example, the storage photon receiver 166 may comprise theentanglement optics 210 of the quantum repeater system 200 of FIG. 3.Further, the pump pulse pathway 164 may extend between the WDM 160 and apump pulse receiver 168. In operation the first and second pump pulsesmay terminate at the pump pulse receiver 168, for example, the pumppulse receiver 168 may comprise a fiber end in embodiments in which thepump pulse pathway 164 comprises an optical fiber.

As depicted in FIG. 1, the quantum memory system 100 may furthercomprise a cooling system 190 thermally coupled to the chalcogenideoptical fiber link 120. As a non-limiting example, the cooling system190 may comprise a cooling chamber and the chalcogenide optical fiberlink 120 may be positioned within the cooling chamber. As anothernon-limiting example, the cooling system 190 may comprise a lasercooling system and the chalcogenide optical fiber link 120 may beoptically coupled to the laser cooling system. It should be understoodthat any cooling system 190 structurally configured to cool thechalcogenide optical fiber link 120 is contemplated.

In operation, the chalcogenide optical fiber link 120 comprising thecore 122 doped with the rare-earth element dopant 130 is structurallyand compositionally configured to absorb and store a storage photonemitted by the storage photon generator 170. For example, when thestorage photon is traversing the chalcogenide optical fiber link 120,the storage photon may transfer an electron of the rare-earth elementdopant 130 from the first split ground state G₁ to the excited energystate E₁, as schematically shown in FIG. 2, to absorb the storagephoton. Further, upon receipt of a first pump pulse output by the pumplaser 180, the first pump pulse may transfer the electron of the of therare-earth element dopant 130 from the excited energy state E₁ into thesecond split ground state G₂, to store the storage photon. Moreover, theoutput from the pump laser may comprise a π-pulse.

Further, the chalcogenide optical fiber link 120 comprising the core 122doped with the rare-earth element dopant 130 is structurally andcompositionally configured to release, on demand, the storage photonstored within the chalcogenide optical fiber link 120. For example, uponreceipt of a second pump pulse output by the pump laser 180, theelectron of the rare-earth element dopant 130 is transferred from thesecond split ground state G₂ back to the excited energy state E₁. Oncein the excited energy state E₁, the electron of the rare-earth elementdopant 130 decays from the excited energy state to the first splitground state G₁, for example, after a decay period, such that thestorage photon exits the photon output end 128 of the chalcogenideoptical fiber link 120.

The decay period comprises a consistent, repeatable time period, thus,upon repeated operation, individual storage photons are released afterthe same decay period. Further, different chalcogenide optical fiberlinks 120 may comprise the same decay period. As a non-limiting example,chalcogenide optical fiber links 120 comprising the same glass anddopant composition may comprise equal decay periods. Thus, a pair ofchalcogenide optical fiber links 120 having equivalent decay periods maybe arranged as the quantum repeater system 200 of FIG. 3 and will eachrelease storage photons simultaneously if they each receive the secondpump pulse simultaneously, to facilitate quantum entanglement of storagephotons using entanglement optics 210 of FIG. 3, as described below.Further, the decay period of individual chalcogenide optical fiber links120 may be determined by performing a photon echo measurement on theindividual chalcogenide optical fiber link 120.

Referring again to FIG. 1, the chalcogenide optical fiber link 120 maycomprise a low phonon energy (e.g., Debye energy), which may limitunintended electron dephasing. Electron dephasing refers to phononassisted coupling from a trapped electron orbital to a degenerate ornearly degenerate orbital. Unintended electron dephasing refers toenergy state transfer (e.g. phonon assisted coupling) into the firstground state G₁ by the electron of the rare-earth element dopant 130,that causes unintentional release of the storage photon before thedesired release of the storage photon. For example, unintended electrondephasing refers to electron dephasing that occurs before receipt of thefirst pump pulse or the second pump pulse by the chalcogenide opticalfiber link 120. Further, lowering unintended electron dephasing mayfacilitate longer photon storage lifetimes and greater photon storageefficiency.

By lowering the phonon energy of the chalcogenide optical fiber link120, the photon storage lifetime and the photon storage efficiency ofthe chalcogenide optical fiber link 120 may be increased. Photon storagelifetime refers to the maximum amount of time a storage photon mayremain stored within the chalcogenide optical fiber link 120 beforeunintended electron decay causes the storage photon to be released.Further, photon storage efficiency refers to the percentage of storagephotons traversing the chalcogenide optical fiber link 120 that areabsorbed and stored. As one non-limiting example, the chalcogenideoptical fiber link 120 having sulfide chalcogenide glass comprises aminimum phonon energy of about 425 cm⁻¹. As another non-limitingexample, the chalcogenide optical fiber link 120 having selenidechalcogenide glass comprises a maximum phonon energy of about 350 cm⁻¹.For comparison, a silicate glass fiber link comprises a maximum phononenergy of as high as about 1100 cm⁻¹. Thus, the quantum memory system100 comprising the chalcogenide optical fiber link 120 may comprise anincreased photon storage lifetime and increased photon storageefficiency.

Moreover, the chalcogenide optical fiber link 120 comprises lowattenuation, increasing the photon storage efficiency. For example, thechalcogenide optical fiber link 120 having sulfide chalcogenide glasscomprises a lower attenuation than the chalcogenide optical fiber link120 having selenide chalcogenide glass along a fiber length of about 1.5μm, because the effective band gap of selenides is shifted to longerwavelengths than the effective band gap of sulfide chalcogenide glass.Thus, sulfide chalcogenide glass may be desirable for chalcogenideoptical fiber links 120 that comprise lengths greater than about 1 μm(due to the lower attenuation) and selenide chalcogenide glass may bedesirable for lengths less than about 1 μm, at least because selenidechalcogenide glass comprises a lower phonon energy than sulfidechalcogenide glass.

As a non-limiting example, the chalcogenide optical fiber link 120 maycomprise between about 35% and about 90% of a chalcogen material (e.g.,sulfur, selenium, tellurium or combinations thereof), and may furthercomprise between about 0% and about 35% Ge and between about 0% andabout 45% As and/or Sb, where the sum of Ge material and As and/or Sbmaterial is between about 10% and about 65%. Additionally thechalcogenide optical fiber link 120 may comprise between about 0.1% andabout 10% Ga and/or In. Further, the chalcogen content of thechalcogenide optical fiber link 120 may comprise between about 50% andabout 150% of the stoichiometric value of the chalcogenide optical fiberlink 120 composition. As one non-limiting example, the chalcogenideoptical fiber link 120 may comprise 25% Ge, 8.33% As, 1.67% Ga, 65% S,and 0.015% of a rare-earth element (e.g., the rare-earth element dopant130). Further, including Ga or like materials within the chalcogenideoptical fiber link 120 may prevent clustering of the rare-earth elementswithin the rare-earth element dopant 130, which reduces spin-spininteractions, thereby reducing unintended electron decay. Moreover, bycooling the chalcogenide optical fiber link 120, for example, using thecooling system 190, the phonon energy of the chalcogenide optical fiberlink 120 may be reduced, increasing the photon storage lifetime and thephoton storage efficiency of the chalcogenide optical fiber link 120.

The chalcogenide optical fiber link 120 doped with the rare-earthelement dopant 130 also comprises a magnetic moment about 3μ_(N) orless, for example, about 1μ_(N) or less. In operation, lower magneticmoments are correlated with increased photon storage lifetime and photonstorage efficiency because chalcogenide optical fiber links 120comprising a low magnetic moment may also comprise low phonon energy,reducing unintended electron decay. As non-limiting example, elementssuch as Y, Sn, and Pb, which each comprise low magnetic moments, mayalso be present in the chalcogenide optical fiber link 120. Further,chalcogenide optical fiber links 120 comprising materials having higheratomic weights may be desired because heavier elements may also compriselower phonon energy.

The chalcogenide optical fiber link 120 doped with the rare-earthelement dopant 130 may also comprise a narrow homogeneous linewidth,which may increase the photon storage lifetime and photon storageefficiencies of the chalcogenide optical fiber link 120 doped with therare-earth element dopant 130. In particular, a narrower homogeneouslinewidth is directly correlated with a longer photon storage lifetime.As used herein, homogeneous linewidth refers the full-width half maximum(FWHM) spectral linewidth of the absorption peak (e.g., wavelength atwhich maximum absorption occurs) of the rare-earth element dopant 130 ofthe chalcogenide optical fiber link 120. The inhomogeneous linewidth thechalcogenide optical fiber link 120 doped with the rare-earth elementdopant 130 may comprise between about 1 nm and about 25 nm, betweenabout 5 nm and 15 nm, or the like, for example, 2 nm, 5 nm, 10 nm, 15nm, 20 nm, or the like. Moreover, the homogeneous linewidth of thechalcogenide optical fiber link 120 doped with the rare-earth elementdopant 130 may comprise about 7.5 MHz or less, for example, 7 MHz, 6MHz, 5 MHz 4 MHz, 3 MHz, 2 MHz, 1 MHz, or the like.

As one non-limiting example, the absorption peak of the chalcogenideoptical fiber link 120 doped with the rare-earth element dopant 130comprising erbium may be between about 1510 nm and about 1550 nm, forexample, between about 1535 nm and about 1545 nm, such as 1540 nm. Asanother non-limiting example, the absorption peak of the chalcogenideoptical fiber link 120 doped with the rare-earth element dopant 130comprising thulium may be between about 1600 nm and about 1700 nm, forexample, between about 1625 nm and about 1675 nm, such as 1660 nm.Further, in operation, the chalcogenide optical fiber link 120 dopedwith the rare-earth element dopant 130 is configured to absorb and storea storage photon traversing the chalcogenide optical fiber link 120, asdescribed above, upon receipt of a first pump pulse output by the pumplaser 180 that comprises a wavelength within 15 nm of the wavelength ofthe absorption peak, for example, within 10 nm, within 5 nm, or equal tothe wavelength of the absorption peak. Further, the chalcogenide opticalfiber link 120 doped with the rare-earth element dopant may release thestorage photon, as described above, upon receipt of a second pump pulseoutput by the pump laser 180 that comprises a wavelength within 15 nm ofthe wavelength of the absorption peak, for example, within 10 nm, within5 nm, or equal to the wavelength of the absorption peak.

The relationship between photon storage lifetime and homogeneouslinewidth may be mathematically described with the following equations:

${\rho_{1} = {{\frac{1}{c}\frac{3}{2{\pi\rho}\; v^{5}\hslash^{4}}{{\langle{\psi_{i}^{el}{V_{1}}\psi_{j}^{el}}\rangle}}^{2}\mspace{14mu} {and}\mspace{14mu} \rho_{2}} = {\frac{1}{c}\frac{3}{2{\pi\rho}\; v^{2}\hslash^{4}}{{\langle{\psi_{i}^{el}{V_{2}}\psi_{i}^{el}}\rangle}}^{2}}}},$

where, V₁ is the first spatial derivative of the crystal field of thechalcogenide optical fiber link 120, V₂ is the second spatial derivativeof the crystal field of the chalcogenide optical fiber link 120, ρ₁ isthe probability amplitude of the transition due to the V₁, ρ₂ is theprobability of the transition due to V₂, c is the speed of light, ρ isthe density of the host material, for example, the chalcogenide opticalfiber link 120, v is the average velocity of a sound wave in thecrystal, ψ_(i) ^(el) is the ground state of the electron (e.g., theelectron of the rare-earth element dopant 130), and ψ_(j) ^(el) is theexcited state of the electron (e.g., the electron of the rare-earthelement dopant 130). Further, a phonon coupling coefficient β_(ij) maybe mathematically described as β_(ij)=

³ cω_(ij) ³ρ₁, where c is the speed of light, ω₁ is the homogeneouslinewidth of the chalcogenide optical fiber link 120, and ρ₁ is the isthe first order probability amplitude of the transition. As shown above,smaller (e.g., narrower) homogeneous linewidths generate smaller photoncoupling coefficients. Further, a small phonon coupling coefficient iscorrelated with low phonon energy and low phonon energy facilitateslonger photon storage lifetimes. Thus, the homogeneous linewidth isinversely proportional to the photon storage lifetime and narrowerhomogeneous linewidth facilitate longer photon storage lifetimes.

Combining the above equations, the homogeneous lifetime may also bemathematically described as

${w_{i}^{\hom}\left( {cm}^{- 1} \right)} = {{{\frac{2\hslash \; {c\left( {kT}_{D} \right)}^{7}}{\pi}\left\lbrack {{\sum_{i \neq j}\frac{p_{1}}{E_{i} - E_{j}}} + p_{2}} \right\rbrack}^{2}\left( \frac{T}{T_{D}} \right)^{7}{\int_{0}^{T_{D}/T}{\frac{x^{6}e^{x}}{\left( {e^{x} - 1} \right)^{2}}{dx}}}} + {p_{1}\left\{ {{\sum_{j < i}{\Delta \; {E_{ij}^{3}\left( \frac{e^{\Delta \; {E_{ij}/{kT}}}}{e^{\Delta \; {E_{ij}/{kT}}} - 1} \right)}}} + {\sum_{j > i}{\Delta \; {E_{ij}^{3}\left( \frac{1}{e^{\Delta \; {E_{ij}/{kT}}} - 1} \right)}}}} \right.}}$

where c is the speed of light, k is the Boltzmann constant, T is thetemperature (e.g., the temperature of the chalcogenide optical fiberlink 120), T_(D) is the Debye temperature, ρ₁ is the first orderprobability amplitude of the transition, ρ₂ is the second orderprobability amplitude of the transition, and E is the energy level.

In some embodiments, the rare-earth element dopant 130 may comprise anon-Kramers rare-earth ion, such as Pr³⁺, Tm³⁺, or the like.Chalcogenide optical fiber links 120 doped with the non-Kramersrare-earth ions may comprise a narrower homogeneous linewidth thanchalcogenide optical fiber link 120 comprising Kramers rare earth ions,for example, due to the lack of Kramers degeneracy of non-Kramersrare-earth ions. This may increase the photon storage lifetime of thechalcogenide optical fiber link 120 and reducing unintended electrondecay. Moreover, when the rare-earth element dopant 130 comprisesthulium, the electrons of the rare-earth element dopant 130 comprisingthulium may split into first and second ground states G₁ and G₂ (FIG. 2)when positioned within a weaker magnetic field than a rare-earth elementdopant 130 comprising erbium.

Referring again to FIGS. 1 and 2, a method of storing and releasing astorage photon using the quantum memory system 100 is contemplated.While the method is described below in a particular order, it should beunderstood that other orders are contemplated. Referring now to FIG. 1,the method may first comprise generating a magnetic field using themagnetic field generation unit 150. As stated above, generating amagnetic field using a magnetic field generation unit 150 causes theground state of electrons of the rare-earth element dopant 130 dopedwithin the core 122 is split into the first ground state G₁ and thesecond ground state G₂, as depicted in FIG. 2.

The method further comprises emitting a storage photon from the storagephoton generator 170 optically coupled to the photon receiving end 126of the chalcogenide optical fiber link 120 and upon receipt of thestorage photon by the chalcogenide optical fiber link 120, the core 122doped with the rare-earth element dopant 130 absorbs the storage photonby transferring an electron of the rare-earth element dopant 130 fromthe first split ground state G₁ to the excited energy state E₁. Next,the method further comprises emitting a first pump pulse from the pumplaser 180 optically coupled to the photon receiving end 126 of thechalcogenide optical fiber link 120 such that the first pump pulsetransfers the electron of the rare-earth element dopant 130 from theexcited energy state to a second split ground state, upon receipt of thefirst pump pulse by the chalcogenide optical fiber link 120, to storethe storage photon within the core 122 doped with the rare-earth elementdopant 130.

Referring still to FIGS. 1 and 2, the method further comprises emittinga second pump pulse from the pump laser 180 such that the second pumppulse transfers the electron of the rare-earth element dopant 130 fromthe second split ground state G₂ to the excited energy state E₁, uponreceipt of the second pump pulse by the chalcogenide optical fiber link120. Once back in the excited energy state E₁, the electron of therare-earth element dopant 130 decays from the excited energy state E₁ tothe first split ground state after a decay period. After the electrondecays into the first split ground state G₁ such that the storage photonexits the photon output end 128 of the chalcogenide optical fiber link120.

Further, in operation, the quantum memory system 100 and moreparticularly, the core 122 of the chalcogenide optical fiber link 120doped with a rare-earth element dopant 130 may absorb and store astorage photon for a photon storage lifetime comprising between about 1ns and about 1 μs, for example, between about 1 ns and about 500 ns orbetween about 1 ns and about 100 ns. Moreover, in operation, the quantummemory system 100 and, more particularly, the core 122 of thechalcogenide optical fiber link 120 doped with a rare-earth elementdopant 130 may absorb and store about 50% or more of a plurality ofstorage photons traversing the chalcogenide optical fiber link 120, forexample, about 70% or more of the plurality of storage photonstraversing the chalcogenide optical fiber link 120, about 90% or more ofthe plurality of storage photons traversing the chalcogenide opticalfiber link 120, or the like.

Referring now to FIG. 3, a quantum repeater system 200 comprising firstand second chalcogenide optical fiber links 220 a, 220 b, one or moremagnetic field generation units 250, first and second storage photongenerators 270 a, 270 b, first and second pump lasers 280 a, 280 b, andentanglement optics 210 is schematically depicted. The first and secondchalcogenide optical fiber links 220 a, 220 b may each comprise thechalcogenide optical fiber link 120 described above with respect to thequantum memory system 100. For example, the first and secondchalcogenide optical fiber links 220 a, 220 b comprise a photonreceiving end 226 a, 226 b opposite the photon output end 228 a, 228 band a core 222 a, 222 b encircled by a cladding layer 224 a, 224 b.Further the cores 222 a, 222 b of the chalcogenide optical fiber links220 a, 220 b may be doped with a rare-earth element dopant 230 a, 230 b,for example, any of the rare-earth element dopants 130 described above.Moreover, the quantum repeater system 200 may further comprise a coolingsystem 290 thermally coupled to each chalcogenide optical fiber link 220a, 220 b. The cooling system 290 may comprise any of the cooling systems190 described above.

The one or more magnetic field generation units 250 may comprise themagnetic field generation units 150 described above. Further, the firstand second chalcogenide optical fiber links 220 a, 220 b are positionedwithin a magnetic field of the one or more magnetic field generationunits 250 when the one or more magnetic field generation units 250generate magnetic fields. Further, while a single magnetic fieldgeneration unit 250 is depicted in FIG. 3, it should be understood thatany number of magnetic field generation units 250 are contemplated. Forexample, the first and second chalcogenide optical fiber links 220 a,220 b may be positioned within the magnetic fields of different magneticfield generation units 250.

Referring still to FIG. 3, the one or more storage photon generators 270a, 270 b are optically coupled to the photon receiving end 226 a, 226 bof each chalcogenide optical fiber link 220 a, 220 b. For example, thefirst storage photon generators 270 a may be optically coupled to thephoton receiving end 226 a of the first chalcogenide optical fiber link220 a and the second storage photon generators 270 b may be opticallycoupled to the photon receiving end 226 b of the chalcogenide opticalfiber link 120 and second chalcogenide optical fiber link 220 b. The oneor more storage photon generators 270 a, 270 b may comprise any of thestorage photon generators 170 described above.

Further, one or more pump lasers 280 a, 280 b are optically coupled tothe photon receiving end 226 of each chalcogenide optical fiber link220. For example, the first pump laser 280 a may be optically coupled tothe photon receiving end 226 a of the first chalcogenide optical fiberlink 220 a and the second pump laser 280 b may be optically coupled tothe photon receiving end 226 b of the chalcogenide optical fiber link120 and second chalcogenide optical fiber link 220 b. The one or morepump lasers 280 a, 280 b may comprise any of the pump lasers 180described above. The one or more storage photon generators 170 a, 170 b,the one or more pump lasers 180 a, 180 b and/or the chalcogenide opticalfiber links 220 a, 220 b may be coupled to one or more alignmentmechanisms 242 to optically align the one or more storage photongenerators 170 a, 170 b and the one or more pump lasers 180 a, 180 bwith the chalcogenide optical fiber links 220 a, 220 b. Further, the oneor more alignment mechanisms 242 may comprise any of the alignmentmechanisms 142 described above. Moreover, the chalcogenide optical fiberlink 220 a, 220 b comprising the core 222 a, 222 b doped with rare-earthelement dopants 230 a, 230 b are configured to absorb and releasestorage photons as described above with respect to FIGS. 1 and 2.

As depicted in FIG. 3, the quantum repeater system 200 may furthercomprise first and second WDMs 260 a, 260 b, positioned adjacent andoptically coupled to the photon output ends 228 a, 228 b of thechalcogenide optical fiber links 220 a, 220 b. The WDMs 260 a, 260 b mayeach comprise the WDM 160 described above. Further, WDMs 260 a, 260 bmay be optically coupled to both storage photon pathways 262 a, 262 band pump pulse pathways 264 a, 264 b for example, the WDMs 260 a, 260 bmay be positioned between the photon output ends 228 a, 228 b and boththe storage photon pathway 162 and the pump pulse pathway 164.

Further, the storage photon pathways 262 a, 262 b may extend between theWDMs 260 a, 260 b and the entanglement optics 210. For example, thefirst storage photon pathway 262 a may extend between and opticallycouple the first WDM 260 a and a first entangling pathway 212 a of theentanglement optics 210. Further, the second storage photon pathway 262b may extend between and optically couple the second WDM 260 b and asecond entangling pathway 212 b of the entanglement optics 210. Further,the pump pulse pathways 264 a, 264 b may extend between the WDMs 260 a,260 b and pump pulse receivers 268 a, 268 b, which may comprise the pumppulse receiver 168, described above

Referring still to FIG. 3, the entanglement optics 210 comprise twoentangling pathways 212 a, 212 b optically coupled to and the photonoutput ends 228 a, 228 b of each chalcogenide optical fiber link 220 a,220 b, for example, using the first and second WDMs 260 a, 260 b and thefirst and second storage photon pathways 262 a, 262 b. The twoentangling pathways 212 a, 212 b may also be optically coupled to twoentanglement detectors 214 a, 214 b. The entanglement optics 210 furthercomprise a beamsplitter 216 positioned such that each entangling pathway212 a, 212 b traverses the beamsplitter 216. Further the twoentanglement detectors 214 a, 214 b may each comprise one or moresingle-photon detectors, e.g., superconducting nanowire single-photondetectors, low noise photodiodes, or the like. In operation, theentanglement optics 210 are structurally configured to entangle pairs ofstorage photons when storage photons output by the photon output ends228 a, 228 b of each chalcogenide optical fiber link 220 a, 220 bsimultaneously traverse the beamsplitter 216.

In operation, each chalcogenide optical fiber link 220 a, 220 bcomprising the core 222 a, 222 b doped with the rare-earth elementdopant 230 a, 230 b is structurally and compositionally configured toabsorb and store individual storage photons emitted by the storagephoton generators 270 a, 270 b. For example, when the storage photonsare traversing each chalcogenide optical fiber link 220 a, 220 b, eachstorage photons may each transfer an electron of the rare-earth elementdopants 230 a, 230 b from the first split ground state G₁ to the excitedenergy state E₁, as schematically shown in FIG. 2, to absorb eachrespective storage photon in each respective chalcogenide optical fiberlink 220 a, 220 b. Further, upon receipt of a first pump pulse output byeach pump laser 280 a, 280 b, each first pump pulse may transfer theelectron of the rare-earth element dopant 230 a, 230 b of eachrespective chalcogenide optical fiber link 220 a, 220 b from the excitedenergy state E₁ into the second split ground state G₂, to store eachstorage photon.

Each chalcogenide optical fiber link 220 a, 220 b comprising the core222 a, 222 b doped with the rare-earth element dopant 230 a, 230 b isalso structurally and compositionally configured to release, on demand,the storage photon stored within each chalcogenide optical fiber link220 a, 220 b. For example, upon receipt of a second pump pulse output byeach pump laser 280 a, 280 b, the electron of the rare-earth elementdopants 230 a, 230 b is transferred from the second split ground stateG₂ back to the excited energy state E₁. Once in the excited energy stateE₁, the electrons of the rare-earth element dopants 230 a, 230 b decayfrom the excited energy state to the first split ground state G₁, forexample, after the decay period, such that the storage photons exit thephoton output ends 228 a, 228 b of the chalcogenide optical fiber link220 a, 220 b.

Further, if the first and second pump lasers 280 a, 280 b emit secondpump pulses that are simultaneously received by the first and secondchalcogenide optical fiber links 220 a, 220 b (e.g., by emitting thesecond pump pulses simultaneously), the first and second chalcogenideoptical fiber links 220 a, 220 b will simultaneously release the storagephotons (after the decay period), allowing the storage photons tosimultaneously traverse the beamsplitter 216 of the entanglement optics210, entangling the storage photons. Moreover, because the chalcogenideoptical fiber links 220 a, 220 b comprise long photon storage lifetimes,one chalcogenide optical fiber link 220 a/220 b may absorb and store afirst storage photon before the other chalcogenide optical fiber link220 a/220 b absorbs and stores a second storage photon, allowing storagephotons that are not simultaneously received by the chalcogenide opticalfiber links 220 a, 220 b to be simultaneously released by thechalcogenide optical fiber links 220 a, 220 b and entangled by theentanglement optics 210.

Referring now to FIG. 4, a non-limiting embodiment of an entangledphoton generator 300 is depicted. The entangled photon generator 300 isstructurally configured to generate four or more entangled photons, forexample, two or more entangled pairs of photons. The entangled photongenerator 300 may be positioned in one or more photon entanglementchains of a quantum key generation system, for example, the quantum keygeneration systems described in U.S. patent application Ser. No.14/680,522. As depicted in FIG. 4, the entangled photon generator 300may comprise first and second quantum repeater systems 310 a, 310 b(each configured to output an entangled pair of photons), entanglementoptics 370, a pathway splitter 375, and an entanglement detector 372.The first and second quantum repeaters systems 310 a, 310 b may compriseany of the quantum repeater systems 200, described above.

In some embodiments, the first and second quantum repeater systems 310a, 310 b may comprise entanglement optics (e.g., the entanglement optics210 of FIG. 3, above) that do not include two entanglement detectors 214a, 214 b such that each entangling pathway 212 a, 212 b may be opticallycoupled to the entanglement optics 370 of the entangled photon generator300. In other embodiments, the first and second quantum repeater system310 a, 310 b may include the entanglement detectors 214 a, 214 b and theentanglement optics 370 of the entangled photon generator 300 may beoptically coupled to each entangling pathway 212 a, 212 b, for example,when the entanglement detectors 214 a, 214 b are structurally configuredto detect the storage photons without the storage photons terminatingtherein.

Referring still to FIG. 4, the entanglement optics 370 may comprise afirst entangling pathway 371 a optically coupled to and extendingbetween a first quantum repeater system 310 a and the entanglementdetector 372 and a second entangling pathway 371 b optically coupled toand extending between a second quantum repeater system 310 b and thepathway splitter 375. Additional entangling pathways 371 arecontemplated in embodiments comprising additional quantum repeatersystems 310. In some embodiments, the entanglement optics 370 furthercomprise a beamsplitter 373 positioned such that each entangling pathway371 a, 371 b traverses the beamsplitter 373. In operation, theentanglement optics 370 are structurally configured to entangle multiplephotons (e.g., storage photons) when the multiple photons simultaneouslytraverse the beamsplitter 373. For example, when each entangled pair ofphotons output by the first and second quantum repeater systems 310 a,310 b simultaneously traverse the beamsplitter 373, all four photons areentangled with each other.

Further, the entanglement optics 370 are configured such that some orall of the entangled photons output by each of the first and secondquantum repeater systems 310 a, 310 b are received by the entanglementdetector 372 and/or the pathway splitter 375. For example, when a firstentangled pair of photons are output by the first quantum repeatersystem 310 a and a second entangled pair of photons are output by thesecond quantum repeater system 310 b and these two entangled pairs ofphotons are entangled with each other at the beamsplitter 373, there isa probability that one of at least three outcomes occur, which aremathematically described by the wave function:

${\Psi\rangle}_{{AA}^{\prime}} = {- {\left\lbrack {{\frac{1}{2}{{2,2}\rangle}} + {\sqrt{\frac{3}{8}}\left( {{{4,0}\rangle} + {{0,4}\rangle}} \right)}} \right\rbrack.}}$

In a first outcome, both the entanglement detector 372 and the pathwaysplitter 375 receive two of the four entangled photons, mathematicallydescribed by the ket |2,2

in the above wave function. In a second outcome, the entanglementdetector 372 receives the four entangled photons, mathematicallydescribed by one of the kets |4,0

or |4,0

in the above wave function. In a third outcome, the pathway splitter 375receives the four entangled photons, mathematically described by one ofthe kets |4,0

or |4,0

in the above wave function. In some embodiments, the probability thatthe pathway splitter 375 receives the four entangled photons is about ⅜.Further, embodiments comprising additional parametric down conversiongenerators are contemplated such that additional entangled pairs ofphotons (e.g., N entangled photons) may be entangled by the entanglementoptics 370. In an embodiment comprising N entangled photons, theprobability that the N entangled photons are received by theentanglement detector 372, the pathway splitter 375, or a combination ofboth is mathematically described by the generalized ket:

$\frac{i^{N}}{{N!}2^{N}}{\sum_{p = 0}^{N}{\begin{pmatrix}N \\p\end{pmatrix}\sqrt{{\left( {2p} \right)!}{\left( {{2N} - {2p}} \right)!}}{{{{2p},{{2N} - {2p}}}\rangle}.}}}$

Further, in some embodiments, at least a portion of both the first andsecond entangling pathways 371 a, 371 b may comprise multicore opticalfibers. For example, a portion of the first entangling pathway 371 athat extends between the beamsplitter 373 and the pathway splitter 375and a portion of the second entangling pathway 371 b that extendsbetween the beamsplitter 373 and the pathway splitter 375 may eachcomprise multicore optical fiber. In some embodiments, at least aportion of both the first and second entangling pathways 371 a, 371 bmay comprise one or more optical waveguides.

In some embodiments, the pathway splitter 375 is structurally configuredto direct entangled pairs of photons into optical fiber links 360optically coupled to the pathway splitter 375. For example, when thepathway splitter 375 receives four entangled photons, the pathwaysplitter 375 may direct two of the four entangled photons into oneoptical fiber link 360 and the pathway splitter 375 may direct two ofthe four entangled photons into another optical fiber link 360. Theoptical fiber links 360 may comprise any optical fiber, for example, aglass optical fiber comprising a single core or comprising multiplecores. Further, in embodiments when the entangled photon generator 300is configured to generate more than four entangled photons, the pathwaysplitter 375 may direct a first subset (e.g., about half) of theentangled photons into one optical fiber link 360 (e.g., a first opticalfiber link) and the pathway splitter 375 may also direct a second subset(e.g., about half) of the entangled photons into another optical fiberlink 360 (e.g., a second optical fiber link). In some embodiments, thepathway splitter 375 may comprise a fused biconical taper splitter, aplanar lightwave circuit splitter, or the like.

In some embodiments, the entanglement detector 372 is structurallyconfigured to measure the number of photons received by the entanglementdetector 372, which also provides information regarding the number ofphotons received by the pathway splitter 375. For example, if twoentangled photons are output by each of the first and second quantumrepeater systems 310 a, 310 b and zero entangled photons are received bythe entanglement detector 372, than all four entangled photons arereceived by the pathway splitter 375. In some embodiments, theentanglement detector 372 may comprise a multi-photon detector. Inalternative embodiments, the entanglement detector 372 may comprise asingle-photon detector, e.g., a superconducting nanowire single-photondetector, a low noise photodiode, or the like.

It is noted that recitations herein of a component of the presentdisclosure being “configured” in a particular way, to embody aparticular property, or to function in a particular manner, arestructural recitations, as opposed to recitations of intended use. Morespecifically, the references herein to the manner in which a componentis “configured” denotes an existing physical condition of the componentand, as such, is to be taken as a definite recitation of the structuralcharacteristics of the component.

For the purposes of describing and defining the present invention it isnoted that the term “about” is utilized herein to represent the inherentdegree of uncertainty that may be attributed to any quantitativecomparison, value, measurement, or other representation. The term“about” is also utilized herein to represent the degree by which aquantitative representation may vary from a stated reference withoutresulting in a change in the basic function of the subject matter atissue.

Having described the subject matter of the present disclosure in detailand by reference to specific embodiments thereof, it is noted that thevarious details disclosed herein should not be taken to imply that thesedetails relate to elements that are essential components of the variousembodiments described herein, even in cases where a particular elementis illustrated in each of the drawings that accompany the presentdescription. Further, it will be apparent that modifications andvariations are possible without departing from the scope of the presentdisclosure, including, but not limited to, embodiments defined in theappended claims. More specifically, although some aspects of the presentdisclosure are identified herein as preferred or particularlyadvantageous, it is contemplated that the present disclosure is notnecessarily limited to these aspects.

It is noted that one or more of the following claims utilize the term“wherein” as a transitional phrase. For the purposes of defining thepresent invention, it is noted that this term is introduced in theclaims as an open-ended transitional phrase that is used to introduce arecitation of a series of characteristics of the structure and should beinterpreted in like manner as the more commonly used open-ended preambleterm “comprising.”

1. A chalcogenide optical fiber link for a quantum communication system,the chalcogenide optical fiber link comprising: a photon receiving endopposite a photon output end; chalcogenide glass comprising sulfur,selenium, and/or tellurium; a core doped with a rare-earth elementdopant; wherein the rare-earth element dopant is configured to absorb astorage photon traversing the chalcogenide optical fiber link (i) whenthe storage photon transfers an electron of the rare-earth elementdopant from a first split ground state to an excited energy state and(ii) when, upon receipt of a laser pump pulse, the laser pump pulsetransfers the electron of the of the rare-earth element dopant from theexcited energy state into a second split ground state; and wherein therare-earth element dopant is configured to release the storage photon(i) when the electron of the of the rare-earth element dopant istransferred from the second split ground state to the excited energystate, upon receipt of another laser pump pulse and (ii) when theelectron of the rare-earth element dopant decays from the excited energystate to the first split ground state such that the storage photon exitsthe photon output end of the chalcogenide optical fiber link.
 2. Thechalcogenide optical fiber link of claim 1, wherein between about 35%and about 90% of molecular weight of the chalcogenide optical fiber linkconsists of sulfur, selenium, tellurium, or a combination thereof. 3.The chalcogenide optical fiber link of claim 1, comprising germanium,arsenic, and/or antimony.
 4. The chalcogenide optical fiber link ofclaim 1, comprising selenide chalcogenide glass.
 5. The chalcogenideoptical fiber link of claim 1, comprising sulfide chalcogenide glass. 6.The chalcogenide optical fiber link of claim 1, wherein the chalcogenideoptical fiber link has a length of at least 0.25 m.
 7. The chalcogenideoptical fiber link of claim 1, wherein the rare-earth element dopantcomprises one or more lanthanide elements.
 8. The chalcogenide opticalfiber link of claim 1, wherein the rare-earth element dopant comprisesscandium and/or yttrium.
 9. A chalcogenide optical fiber link for aquantum communication system, the chalcogenide optical fiber linkcomprising: a photon receiving end opposite a photon output end;chalcogenide glass; a core doped with a rare-earth element dopant;wherein the rare-earth element dopant comprises an inhomogeneouslinewidth of about 25 nm or less and/or a homogeneous linewidth of about5 MHz or less; wherein the rare-earth element dopant is configured toabsorb a storage photon traversing the chalcogenide optical fiber link(i) when the storage photon transfers an electron of the rare-earthelement dopant from a first split ground state to an excited energystate and (ii) when, upon receipt of a laser pump pulse, the laser pumppulse transfers the electron of the of the rare-earth element dopantfrom the excited energy state into a second split ground state; andwherein the rare-earth element dopant is configured to release thestorage photon (i) when the electron of the of the rare-earth elementdopant is transferred from the second split ground state to the excitedenergy state, upon receipt of another laser pump pulse and (ii) when theelectron of the rare-earth element dopant decays from the excited energystate to the first split ground state such that the storage photon exitsthe photon output end of the chalcogenide optical fiber link.
 10. Thechalcogenide optical fiber link of claim 9, wherein between about 35%and about 90% of total molecular weight of the chalcogenide opticalfiber link consists of one or more of sulfur, selenium, tellurium, or acombination thereof.
 11. The chalcogenide optical fiber link of claim 9,comprising germanium, arsenic, and/or antimony.
 12. The chalcogenideoptical fiber link of claim 9, comprising selenide chalcogenide glass.13. The chalcogenide optical fiber link of claim 9, comprising sulfidechalcogenide glass.
 14. The chalcogenide optical fiber link of claim 9,wherein the chalcogenide optical fiber link has a length of at least0.25 m.
 15. The chalcogenide optical fiber link of claim 9, wherein therare-earth element dopant comprises one or more lanthanide elements. 16.The chalcogenide optical fiber link of claim 9, wherein the rare-earthelement dopant comprises scandium and/or yttrium.
 17. A chalcogenideoptical fiber link for a quantum communication system, the chalcogenideoptical fiber link comprising: a photon receiving end opposite a photonoutput end; chalcogenide glass; a core doped with a rare-earth elementdopant, wherein the rare-earth dopant comprises erbium, thulium,praseodymium, or a combination thereof; wherein the rare-earth elementdopant is configured to absorb a storage photon traversing thechalcogenide optical fiber link (i) when the storage photon transfers anelectron of the rare-earth element dopant from a first split groundstate to an excited energy state and (ii) when, upon receipt of a laserpump pulse, the laser pump pulse transfers the electron of the of therare-earth element dopant from the excited energy state into a secondsplit ground state; and wherein the rare-earth element dopant isconfigured to release the storage photon (i) when the electron of the ofthe rare-earth element dopant is transferred from the second splitground state to the excited energy state, upon receipt of another laserpump pulse and (ii) when the electron of the rare-earth element dopantdecays from the excited energy state to the first split ground statesuch that the storage photon exits the photon output end of thechalcogenide optical fiber link.
 18. The chalcogenide optical fiber linkof claim 17, wherein between about 35% and about 90% of total molecularweight of the chalcogenide optical fiber link consists of one or more ofsulfur, selenium, tellurium, or a combination thereof.
 19. Thechalcogenide optical fiber link of claim 17, comprising selenidechalcogenide glass.
 20. The chalcogenide optical fiber link of claim 17,comprising sulfide chalcogenide glass.